Traditional retrieval engines (such as Web Search Engines and Corporate Search Engines) employ an index, which contains a collection of documents (such as files, web pages, etc.) stored in such a way as to make the retrieval of documents efficient when a user query arrives.
Traditional retrieval engines function as follows: the engine relieves a query, and retrieves some documents from the index that are considered relevant to the query (for example, those that match the keywords in the query).
This requires that the index store a description of all the documents before the retrieval phase. For example, when issuing a query to a web search engine, the web search engine index contains a description (such as the URL and title) of all the web documents that can be retrieved. This provides very high efficiency (e.g. speedy of retrieval) but has the very high storage cost of having to store all the documents in the index prior to the retrieval.